1. Field of the Invention
This invention relates to pressure-sensitive recording sheets. More particularly, this invention relates to pressure-sensitive developing sheets which can provide recorded black color images having a high color intensity and excellent light fastness.
2. Prior Art
Generally, pressure-sensitive recording sheets are composed of a top sheet coated on its backside with microcapsules containing electron-donating colorless or light-colored color-forming dyes dissolved in organic solvent (capsule oil) and an under sheet having on its front side a developing layer containing electron-accepting developing agents. These two sheets are laid so that the two coated surfaces are faced with each other and arranged so that the printed records are produced in such a way that when pressure is applied with ball-point pen or typewriter, the capsules in the pressed area are ruptured and the emerged capsule oil containing color-forming dyes is transferred to the developing layer and brings about color-forming reaction. Moreover, a plurality of copies can be obtained by using intermediate sheets each of which has a developing layer on its front side and microcapsules on its back side, sandwiched between the top and under sheets. Accordingly, the developing sheets of this invention include intermediate sheets in addition to the above-mentioned under sheet.
The developing agents which are conventionally known include inorganic solid acids such as activated clay (shown in, for example, the Japanese Patent Publication No. 7622/1966), attapulgite (shown in, for example, the U.S. Pat. No. 2,712,507), substituted phenols and diphenols (shown in, for example, the Japanese Patent Publication No. 9309/1965), p-substituted phenol/formaldehyde polymers (shown in, for example, the Japanese Patent Publication No. 20144/1967), aromatic carboxylic acid metal salts (shown in, for example, the Japanese Patent Publication No. 10856/1974), and 2,2'-bisphenol sulfone compounds (shown in, for example, the Japanese Patent Laid-open No. 106313/1979).
Of these developing agents, the activated clay is prepared by treating Japanese acid clay or the like with a mineral acid to elute iron or other basic components and increase the specific surface area. The activated clay having a specific surface area of not less than 200 m.sup.2 /g is thought to have a particularly excellent developing effect. Such an activated clay is advantageous in that it is very inexpensive as compared with those of the above-mentioned organic developing agents.
With respect to the color of a developed color image of a pressure-sensitive manifold sheet, blue color-forming manifold sheets prepared by using, as a developing agent, Crystal Violet Lactone (CVL), Benzoyl Leuco Methylene Blue (BLMB) or the like were initially used. However, as demands of the market for black-color forming manifold sheets grew greater, a method for mixing at least two dyes having different developed colors has been adopted in order to produce black color recorded images, as disclosed, for example, in the Japanese Patent Publication Nos. 4698/1970 and 4614/1971. For example, the black color-forming dyes can be obtained by combining dyes having developed colors which are in a relationship of complementary colors or which constitute primary colors such as blue/yellowish orange, blue/yellow/yellowish orange/red, and blue/green/red.
This system is chiefly applied to pressure-sensitive manifold sheets in which an inorganic solid acid such as activated clay or attapulgite is used. However, because the dyes have different rates of color formation and different fastness to the light, temperature or moisture, they have a drawback that the developed color changes with the lapse of time when the images are further exposed to the sunlight or ultraviolet rays during the period from the initial color formation to the final color formation or when they are stored for a long time. Furthermore, the following problems are frequently encountered: the color mixing and matching requires much time; because many kinds of dyes are used in quantities, the cost of sheets become higher; and the dissolution of dyes in a solvent (capsule oil) shall be difficult.
In order to eliminate these drawbacks, the fluoran dyes which can form a black color by itself have been developed.
Exemplary of these dyes are 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N-cyclohexly-N-methyl-amino)-6-methyl-7-anilonofluoran, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)-fluoran and 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran. These dyes have an advantage that when the organic developing agent such as p-substituted phenol/formaldehyde polymer, aromatic carboxylic acid metal salt or 2,2'-bisphenol sulfone compound metal salt is used as a developing agent, the above dyes alone or in combination with small amount of blue, red or the like color dyes acting as a complementary color dye can produce black developed color images whose color changes little with the lapse of time.
On the other hand, these dyes have disadvantages in that when they are used in combination with inorganic developing agent such as activated clay, the reddish black or greenish black color appears, and when they are exposed to the sunlight or ultraviolet rays, the developed images discolor or fade to reddish brown, as a result, the developed images with a stable color cannot be obtained.
Accordingly, although the activated clay is inexpensive and has a superior developing performance as compared with organic developing agents, there has been no choice but to adopt the above-mentioned mixed dye system in order to obtain a black color on clay type developer.